Satellites placed in the geostationary orbit appear stationary seen from earth. This enables communication between earth stations (in the following referred to as terminals or VSAT terminals) using highly directive, fixed antennas that are pointed to the geostationary orbit of the satellite of interest. The transmitting terminal sends a polarized high radio frequency signal (RF) to the satellite, where the signal is received, shifted in frequency, amplified and retransmitted on the opposite polarization to the receiving terminal.
The polarization in the satellite is normally opposite for transmission and reception. The polarization schemes are either circular (left-hand and right-hand) or linear (vertical or horizontal). E.g. one system can communicate by transmitting vertical polarization and receiving horizontal polarization while a different system may use opposite polarizations for the same frequencies.
Due to the high directional gain of the satellite dish, as well as the discrimination achieved by the polarization, the same frequency band can be re-used without any further frequency planning considerations:                In the same satellite, the opposite polarization can be re-used        Neighbour satellites at sufficient spacing in the geostationary orbit (this depends on the frequency band) can re-use the same frequency band on both polarizations        
A fundamental assumption for the uncoordinated frequency re-use described above is that terminals comply with agreed levels of unwanted transmission that may cause interference:                Crosspolar Interference—unwanted transmission on opposite polarization.        Adjacent Satellite interference—unwanted transmission towards neighbour satellites        In the following, both cross polar and adjacent satellite inference is referred to as interference, and the term interfered link refers to both cross-polar and adjacent satellite links or spectrum where interference occurs.        
Interference levels are managed by the satellite segment operators by only allowing antennas with accepted properties with respect to:                Transmission level on opposite polarization when the antenna is correctly aligned        Transmission level (gain) as a function of the offset angle (angle between the measured direction and the antenna main pointing direction), ensuring low transmission to neighbour satellites when the antenna is correctly aligned        Approved alignment procedures        It is essential that the antenna is properly aligned. The alignment is normally monitored by the satellite operator NCC (Network Control Centre) when the terminal is taken into use (“commissioning”), ensuring that the terminal antenna is installed correctly and that the transmission is as planned:        Correct power level        Correct pointing        Acceptable crosspolar interference level        The interfering power level to the neighbour satellites is not normally measured as the procedure and antenna properties shall ensure that the radiation outside the main beam is below specified levels        
Occasionally the terminal antenna may come out of alignment, e.g. if it has been displaced by strong winds. A misaligned antenna may cause interference:                Adjacent satellite interference        The satellite in the neighbouring orbital position receives an increased level of the transmitted signal as interference, disturbing services        Crosspolar interference        The misalignment causes increased transmission level of the opposite polarization, disturbing services in the crosspolar segment        
Such interferences are generally a violation of the agreed operational terms for using the space segment. In some cases the terminal should be re-aligned as soon as possible within a reasonable time window, while in other cases the terminal should be taken out of service at once until the antenna has been re-aligned. In other cases equipment failures cause interference to occur. However, it is not straightforward to identify the terminal causing the interference with currently available technology.
For terminals transmitting a continuous signal, there is a single source for the cause and the terminal causing significant and disturbing interference can normally be identified by the carrier frequency itself.
In the case of satellite communication systems with many terminals sharing the same satellite resources, such as in VSAT networks (VSAT=Very Small Aperture Terminals), finding the terminal causing interference is more complicated. In VSAT networks, the same frequency resources are shared in time division mode, TDMA. In these networks, the terminal requests capacity and the network master (hub station) dynamically allocates time slots (DAMA—Demand Assignment Multiple Access) to the terminals. Each time slot is assigned to one terminal only by signalling messages broadcast on the forward link (hub to terminal). Each terminal in the network has a unique ID that the VSAT network operator assigns. The unique terminal ID may be 1) extracted from the burst transmitted to the hub on the return link (terminal to hub) by demodulating and decoding the burst or 2) by analysing information broadcasted from the hub to the terminals, e.g. signalling of assignment of time slots to each terminal.
The interference will only occur in the time slots assigned to the interfering terminal. However, synchronising the information found in the burst time plan with bursts detected in the TDMA channel remains a problem with current solutions.
The satellite owner monitors the satellite space segment continuously from a control centre operated 24 hours a day. Detecting the occurrence of interference is currently done using power density spectrum monitoring systems. The limitation of this approach is that it can only detect the interference level when it exceeds the allowed interference level. Also, the interference may not be possible to observe if other services are in operation in the same frequency segment.
VSAT terminal interference is especially complicated and time-consuming to resolve as there is currently no tool available to identify the source of interference with certainty.
Also, there is currently no general solution which allows measurement of the interference level at installation and commissioning time, or which allows the normal interference level to be measured under normal operational conditions.
To identify terminals causing interference, the normal procedure is to close down or move the possible terminals (or groups of terminals) one-by-one until the culprit is found. However, such a procedure is generally very difficult to perform for commercial reasons as it disturbs the network operations.
If the interference is large, it may be possible to demodulate and decode the information of the interfered burst in a direct manner. This approach is covered by basic TDMA link reception and existing well-known technologies and is outside the scope for this invention.
From US 2005/0085186 A1 there is known a method of detecting if terminals are causing interference between adjacent satellites. The terminals are sending on separate carrier frequencies, according to a fixed plan. A station is detecting the power spectrum of signals received from an adjacent satellite to uncover possible interference. The frequency of an interfering carrier is used to identify the interfering terminal.